A method for determining a starting gear step for a vehicle having a stepped variable transmission with several gear steps.
Depending on load and driving mode, different gear steps are adequate for starting of which one gear step is selected as starting gear step, particularly in heavy commercial vehicles with multi-speed stepped variable transmissions.
For an optimum start on a level road, e.g. it can be advantageous in an unloaded commercial vehicle having a 16-gear transmission to select the fifth gear as the starting step, while in a fully loaded vehicle on an inclined road, it may be necessary to start in the first or second gear. In a driving situation, if too low a starting gear step is selected, the consequence is several unnecessary gear changes. The starting operation will last longer if the selected starting gear step is too high, with the consequence of a high starting clutch load, or it has to be absolutely interrupted again when available traction force is not sufficient in the selected starting gear.
In vehicles with conventional mechanical stepped variable transmissions the driver selects the starting gear step, while methods for determining a starting gear step preferably find application in automated stepped variable transmissions and power shiftable automatic transmission. It is possible to let the suitable starting gear steps be determined by an electronic control device and suggested to the driver, e.g. via a display. Such a display can also be helpful for the driver also in a vehicle having a conventional mechanical stepped variable transmission. It is of course also possible to have this calculated starting gear automatically introduced by an automated stepped transmission or power shiftable transmission when the vehicle reaches a standstill.
U.S. Pat. No. 5,406,862 discloses a method for determining a starting gear step where the starting gear step is determined by an electronic control device according to the vehicle mass and the gradient of the road. The road gradient dependent on the driving mode is measured with an inclination sensor while the vehicle mass is determined according to the gradient of the road and acceleration of the vehicle.
The suitable starting gear step is stored in a characteristic field memory according to vehicle mass and road gradient.
On the basis of this prior art, the problem of the invention is to further develop a method for determining a starting gear step wherein no inclination sensor is needed.
While in the known method cited above the climbing resistance, which is a variable dependent on the driving mode, is determined by means of an inclination sensor, according to the invention, depending on the driving condition, the variable dependent on the driving mode is determined from the vehicle mass, an actual traction and an actual vehicle acceleration resulting therefrom while the vehicle moves.
In an advantageous development of the method the variable dependent on the driving mode comprises the climbing resistance and the rolling resistance of the vehicle during the starting operation. Hereby the positive tractional resistance is better reproduced which, in the starting operation, is composed mainly of the climbing resistance and the rolling resistance of the vehicle. The air resistance can be disregarded in the starting operation.
The tractional resistance, which is equal to the sum of rolling and climbing resistances, can be calculated by solving the movement equation from the variables: vehicle mass MFzg, actual traction Fzug and vehicle accelerationxcex1Fzg.
The vehicle mass can be indicated by the driver or also measured with sensors in the running gear. In particular it is advantageously determined by a method in which, using commonly available rotational speed sensors during two measurements offset in time within a measuring period, a traction variable and a movement variable resulting therefrom are respectively determined.
EP 0 666 435 A2 discloses such a method which comprises two measurements of the traction produced by the prime mover offset in time and the acceleration resulting therefrom. It is assumed that the unknown tractional resistance in two consecutive measurements is substantially the same for both measurements so that the unknown variable can be abridged. In this known method both during a traction phase and during a traction-free phase, while one clutch is opened for a gear shift of a stepped variable transmission, a value of a gear torque and a value of a vehicle acceleration respectively are determined, wherefrom the actual mass of the vehicle can be calculated.
It has proven to be especially advantageous if the first of the two measurements comprises a first data acquisition period, the second of the two measurements a second data acquisition period, the duration of both data acquisition periods is longer than a minimum duration, the traction variable corresponds to the timed integral of the traction acting during the respective data acquisition period, and the movement variable corresponds to the change of speed of the vehicle occurring during the respective data acquisition period. If this method is made the basis for determining the mass, the starting gear step can be determined without the need of an additional sensor.
The method for determining the mass is based on the following equation:       M    Fzg    =                              ∫                      t            0                                t            1                          ⁢                              F            Zug                    ⁢                      xe2x80x83                    ⁢                      ⅆ            t                              -                        M          Gang                ⁢                  xe2x80x83                ⁢                  (                                    v              1                        -                          v              0                                )                                    v        1            -              v        0            -              v        3            +              v        2            
wherein:
MFzg the vehicle mass to be determined in kg,
Fzug the traction or the engine torque calculated for the gear in N,
Mgang a correcting variable corresponding to the sum of the inertia torques of the engine, clutch and stepped variable transmission, reduced to the transitory movement of the vehicle in kg,
t0, t1 initial and end moments of the traction phase,
V0, V1 speeds of the vehicle at the beginning and end of the traction phase in m/s,
V2, V3 speeds at the beginning and end of the traction-free phase in m/s.
At this point reference is made to the Applicant""s unpublished older patent application, the object of which is a method for determining the mass. The contents of the application belong to the disclosure content of the instant application.
In one embodiment of the inventive method the starting gear step is determined according to an engine torque available for starting, and to a maximum slip time and/or to a maximum friction work of the starting clutch during the starting operation. To this end, there are first determined during the starting operation the engine torque available, a maximum admissible value of a slip time and/or a maximum admissible value for a friction work of the starting clutch. Beginning with a highest gear step adequate for starting it is then predetermined in a calculation loop depending on the ratio of the gear step, the vehicle mass, the tractional resistance, an engine torque available for starting, and values for the slip time and/or the friction work. These predetermined values are compared with the previously determined maximum admissible values. The calculation loop is repeated with the next lower gear step adequate for starting until a gear step is found in which the predetermined values are smaller than or equal to the maximum admissible values. The gear step is issued as the starting gear step.
The maximum admissible values for the slip time and/or the friction work are advantageously stored in the electronic control devicexe2x80x94preferably in a characteristic field memoryxe2x80x94according to the road gradient, the vehicle mass and/or the load state of the clutch, which takes into account the loading history of the clutch. Hereby a great assurance is obtained related to maladjustments which can lead to damages in the starting clutch. Likewise the engine torque available for starting can also be stored, according to the road gradient and/or the vehicle mass; not in all situations must a start occur below full engine load.